MICROSCOPIC ILLUSTRATIONS. 295 



drilled, so as to receive the tempered steel pin, which will 

 thus be in the line of the axis of the ball and socket, which 

 will also be in that of the pillar, when the body is truly 

 horizontal. The milled head, which lets down to steady 

 the legs, must be removed, and in its place a piece of brass, 

 armed with a point, screwed on, to mark the apex of the 

 angle on a piece of paper placed under the instrument; 

 the angle of aperture is then measured upon the same 

 principle as in the instrument already described, by attach- 

 ing the diagonal camera lucida to the eye-piece, and marking 

 the legs of the angle on the paper to be afterwards mea- 

 sured with a protractor. 



The method here given for appreciating the quality of 

 microscopes applies also to all erecting eye-pieces having 

 an external focus in front of the bottom glass, such eye- 

 pieces being in fact neither more nor less than compound 

 microscopes, or engiscopes. I have called it exact, and 

 prodigiously exact it must be, for it is quite capable of 

 rectifying some of our modern optical theorists. No working 

 optician CB.TI finish an instrument unless he is acquainted 

 with it, and it will be seen whether erecting eye-pieces, &c. 

 can be made achromatic with only three lenses; whether 

 their compensation for colour can take place any how, and 

 whether it must not exist both in that part which forms 

 the image, and in that which views it, to produce achro- 

 matism on the whole ; and whether their spherical aber- 

 ration can be reduced so low with the naked aperture of 

 the bottom glasses as analysis would lead us to believe. If 

 the curves of an object-glass are calculated falsely, and 

 give an excess of aberration either to the concave or con- 

 vex lenses, it will be instantly detected. For details on 

 Eye-pieces, see " Micrographia," Chapter 6. 



